Lithium secondary battery

ABSTRACT

A lithium secondary battery using lithium manganese oxide as a positive active material and having excellent charge and discharge cycle properties. 
     As a positive active material of a lithium secondary battery, lithium manganese oxide having a cubic spinel structure, in which the strength ratio (P 2 /P 1  strength ratio) of the primary endothermal peak (P 1 ) appearing around 950° C. and the secondary endothermal peak (P 2 ) appearing around 1100°0 C. in differential thermal analysis is under 1, is used.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/613,127, filed Jul. 10, 2000, now abandoned the entirety ofwhich is incorporated herein by reference.

This application claims priority from Japanese Application No.11-216794, filed in Japan on Jul. 30, 1999, the entirety of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a lithium secondary battery, in which apositive active material is lithium manganese oxide, which has excellentcharging and discharging cycle properties.

In recent years, miniaturization and reduction of weight has beenaccelerated for such portable electronic instruments as a portablephone, a VTR, and a note-type PC. As a cell for an electric powersource, a secondary battery, in which complex oxide made of lithium anda transition element, carbon material, and an organic electrolytesolution prepared by dissolving an Li ion electrolyte in an organicsolvent are used for the positive active material, negative activematerial, and the electrolyte, respectively, is being used.

Such battery is normally named as lithium secondary battery or lithiumion battery and has features of a high energy density and a highsingle-cell voltage as about 4 V. Thus, the lithium secondary batteryattracts attention as an electric power source of not only said portableelectronic instruments, but also electric vehicles (hereafter, EV) orhybrid electric vehicles (hereafter HEV) which are expected to becomepopular to the public as a low pollution automobile on the basis of thelatest environmental problems.

In such a lithium secondary battery, cell capacity and charge anddischarge cycle properties (hereafter, cycle properties) largely dependon material characteristics of the positive active material used. Here,the complex oxide made of lithium and a transition element used as thepositive active material is specifically exemplified by lithium cobaltoxide (LiCoO₂), lithium nickel oxide (LiNiO₂), and lithium manganeseoxide (LiMn₂O₄) or the like.

SUMMARY OF THE INVENTION

Among them, lithium manganese oxide spinel (stoichiometric composition:LiMn₂O₄) has features of a low raw-material cost, a large outputdensity, and a high electric potential. On the other hand, the followingis a problematic defect: discharge capacity gradually decreases ascharge and discharge cycles are repeated, thus it is difficult to yieldgood cycle properties.

Li ions are released from the positive active material or inserted inthe positive active material by charge or discharge. Here, crystallinestructural change of the positive active material becomes irreversible.Thus, apart of Li ions do not contribute to battery reaction after arepeat of charge and discharge cycles. This may be a major cause of areduction in discharge capacity.

Therefore, the inventors carried out various studies to intend tostabilize the crystal structure of lithium manganese oxide spinel. As aresult, we found that the cycle property is improved when lithiummanganese oxide spinel having a given thermal property is used for thepositive active material. We have established a manufacturing method forlithium manganese oxide spinel having such a thermal property.

The present invention provides a lithium secondary battery,characterized in that lithium manganese oxide is used as a positiveactive material having a cubic spinel structure of which strength ratio(P₂/P₁ strength ratio) of a primary endothermal peak (P₁) appearingaround 950° C. and a secondary endothermal peak (P₂) appearing around1100° C. in differential thermal analysis is under 1.

Here, it is more preferable that said strength ratio is lower than 0.5.It is also preferable that Li/Mn ratio is over 0.5 in lithium manganeseoxide. Such lithium manganese oxide can be yielded by firing a mixtureof salt and/or oxide of respective elements adjusted to a givenproportion in an oxidation atmosphere, under a temperature in the rangeof 650 to 1000° C., and for a duration between 5 hours and 50 hours. Insuch a manufacturing method, firing is preferably carried out twice ormore. Preferably, firing temperature is gradually increased as thenumber of times of firing increases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a graph showing the result of thermal analysis for Example1 and FIG. 1(b) is for the Comparative Example, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In the lithium secondary battery according to the present invention,lithium manganese oxide, which has a cubic spinel structure, (hereafter,simply using lithium manganese oxide) is used as the positive activematerial. Here, a stoichiometric composition is represented by LiMn₂O₄.However, the present invention is not restricted to lithium manganeseoxide having such a stoichiometric composition, but those, in which apart of Mn, a transition element, is substituted by other one or moreelements M, is also preferably used.

When a part of Mn of lithium manganese oxide LiMn₂O₄ with astoichiometric composition is substituted by one element M, the generalformula is expressed by LiM_(x)Mn_(2−x)O₄ (X represents a substitutedamount). When such element substitution is carried out, the Li/Mn ratio(molar ratio) is (1+X)/(2−X) in the case where Mn is substituted by Liand Li is excessive, and 1/(2−X) in the case where Mn is substituted byan element M other than Li. In either case, always the Li/Mn ratio >0.5is held. Thus, according to the present invention, those showing theLi/Mn ratio over 0.5 are more preferably used.

When a part of Mn of lithium manganese oxide LiMn₂O₄ according tostoichiometric composition is substituted by two or more elements M₁,M₂, . . . , M_(m), the general formula is expressed by Li (M_(1(x1)),M_(2(x2)), M_(m(xn)))_(x)Mn_(2−x)O₄ (X represents a substituted amount;a sum of X1 to Xn is 1). Also in this case, the Li/Mn ratio >0.5 isheld. Thus, according to the present invention, substitution of Mn by aplurality of, two or more, elements is more preferable.

The substitution element M is exemplified by Li, Fe, Mn, Ni, Mg, Zn, B,Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W. Here, of thesesubstitution elements M, theoretically, Li becomes +1 valent, Fe, Mn,Ni, Mg, and Zn are +2 valent, B, Al, Co, and Cr are +3 valent, Si, Ti,and Sn are +4 valent, P, V, Sb, Nb, and Ta are +5 valent, and Mo and Ware +6 valent ions. All these elements show a mixed crystal in LiMn₂O₄.Co and Sn may be +2 valent, Fe, Sb, and Ti may be +3 valent, Mn may be+3 and +4 valent, and Cr may be +4 and +6 valent.

Therefore, various species of substitution element M may exist in acondition having a mixed valence. The amount of oxygen is notnecessarily 4 as expressed by theoretical chemical composition and mayexist in defect or excessive condition in a range of maintaining thecrystal structure.

According to the present invention, lithium manganese oxide havingvarious compositions as described above and showing characteristics, inwhich strength ratio (P₂/P₁ strength ratio) of a primary endothermalpeak (P₁) appearing around 950° C. and a secondary endothermal peak (P₂)appearing around 1100° C. is under 1 in differential thermal analysis,used as the positive active material. These endothermic peaks P₁ and P₂are considered to show a phase change of lithium manganese oxide.

The differential thermal analysis is a method to detect chemical changesaccompanying thermal transfer, which occurs in a sample according to arise in temperature, on the basis of endothermic peak and exothermicpeak. Normally, the measurement is conducted simultaneously withthermogravimetric analysis by using a thermobalance.

The differential thermal analysis can be conducted in various gasatmospheres by sending a given gas to the sample or mounting theapparatus itself in a glove box. The condition of measurement of thedifferential thermal analysis according to the present invention isbased on an air atmosphere (simply, atmosphere) under an atmosphericpressure.

Concerning lithium manganese oxide, there is, for example, a report aspublished on “Atsuo Yamada et al. (July, 1995) J. Electrochem. Soc. Vol.142, No. 7, p. 2149-2156. Synthesis and Structural Aspects of LiMn₂O₄ ±δas a Positive Electrode for Rechargeable Lithium Batteries,” in thatvarious change in oxygen concentration in a measurement atmosphereallowed a change in shape of a curve yielded from the differentialthermal analysis and the shift or disappearance of a position of theendothermic peak regarded as showing the same chemical change.

If the atmosphere of the gas to be measured is not constant, the primaryendothermal peak and the secondary endothermal peak cannot be defined.Therefore, the measurement in the present invention is conductedprincipally in an atmosphere. Meanwhile, the appearance of the primaryendothermal peak and the secondary endothermal peak of the presentinvention are observed at least in a temperature rising process in thedifferential thermal analysis.

As a rule, in a case of a given rate of temperature rise of a sample inwhich an apex of the endothermal peak occurs at 950° C., whenmeasurement is carried out in the rate of temperature rise lower thanthat of the given rate of temperature rise, the endothermal peak shiftsto a higher temperature. In contrast, when measurement is carried out inthe rate of temperature rise higher than the given rate of temperaturerise, the endothermal peak shifts to a lower temperature. As the resultof the differential thermal analysis, the rate of temperature riseshould also be determined in order to define the positions of theprimary endothermal peak and the secondary endothermal peak, because theposition, where the endothermal peak occurs, shifts according to therate of temperature rise in the differential thermal analysis.

Therefore, in the present invention, the primary endothermal peak isdefined as an endothermal peak appearing around 950° C. and thesecondary endothermal peak is defined as an endothermal peak appearingaround 1100° C., in the case where the temperature rising rate is 5°C./min. The term “around” is used for description in consideration ofthe shift in the peak position according to the temperature rising rateas described above. The condition of the peak strength ratio accordingto the present invention applied between two endothermal peaks appearingin the case where the temperature rising rate is 5° C./min. can benaturally applied even when endothermal peak position shifts afterchanging the temperature rising rate.

In the case where P₂/P₁ strength ratio, which is a strength ratio of theprimary endothermal peak (P₁) and the secondary endothermal peak (P₂)measured under the given condition as described above, in other terms, aratio yielded by dividing P₂ strength by P₁ strength, is lower than 1, agood cycle property is yielded as shown in an example described later.The peak strength is not a peak area, but a distance from a base line tothe apex of the peak; more specifically, the length of a normal from theapex of the endothermal peak to a line, when the point of starting therise of the endothermal peak is connected to the point of the end of thepeak with the line.

The differential thermal analysis curve for lithium manganese oxide with20% oxygen concentration described in “J. Electrochem. Soc.” previouslycited is very similar to the differential thermal analysis curve of anexample described hereinafter; the P₂ strength is almost equal to the P₁strength. It is shown that although the conventional lithium manganeseoxide showing such peak strength scarcely differs in the P₁ strengthfrom lithium manganese oxide satisfying the requirement of the peakstrength according to the present invention, there is a great differencein the P₂ strength. From the comparison of examples with ComparativeExamples as described later, it is considered that lithium manganeseoxide with a small P₂ strength shows a stable crystal structure and agood cycle property.

The following description is for the method of synthesizing lithiummanganese oxide having thermal properties as described above. Salt(s)and/or oxide(s) of respective elements (including a substituting elementM in the case where element substitution is carried out) are used as rawmaterials of synthesis. The salts of respective elements are notspecially restricted, but those having a high purity and requiring a lowcost are preferably used as the raw materials. It is preferable to usecarbonates, hydroxides, organic acid salts which generate no harmfuldecomposition gases in temperature rise and firing. However, nitrates,hydrochlorides, and sulfates are also usable. Among raw materials of Li,Li₂O, that is an oxide, is strongly hygroscopic and difficult to handle,and thus its chemically stable carbonate is preferably used.

A mixture of such raw materials in a given proportion is first burnt inan oxidization atmosphere, under a temperature ranging from 650° C. to1000° C. for 5 to 50 hours. Here, an oxidization atmosphere is,generally, the atmosphere having an oxygen partial pressure under whichthe sample in a furnace normally initiates an oxidization reaction.Specifically, the atmosphere and an oxygen atmosphere are those.

Homogeneity of the composition is not always good after the firstfiring. In the case where Li/Mn ratio >0.5 is satisfied, i.e., elementsubstitution is carried out for Mn in the stoichiometric composition, itwas found experimentally that a product having a given thermal propertybecomes easy to yield by one firing operation particularly for acomposition with excessive Li prepared by substitution of a part of Mnby Li, Ti, and Mg. The reason for it is not clear, but it is assumedthat crystal structure is stabilized by the addition of the substitutionelement M.

As described above, a part of categories of compositions allowssynthesis of lithium manganese oxide showing the given thermal propertyonly by one firing. However, preferably, firing is separately carriedout in a plurality of times in order to establish a synthesis conditionnot influenced by composition.

The number of times of firing greatly depends on firing temperature andfiring duration. Larger times of firing are necessary for a lower firingtemperature and/or a shorter firing duration. According to species ofthe substitution element M, the number of times of firing may bepreferably increased in view of making an even composition. This case isthat addition of the substitution element M does not allow forming phaseatmosphere suitable for the growth of crystals.

However, increase in the number of times of firing requires a longerproduction process. Therefore, it is preferable that the number of timesof firing is limited to the least necessary. In comparison with thesample yielded by firing once, the sample yielded by a plurality oftimes of such firing shows a sharp, projected peak on XRD chart. By thisfact, improved crystallinity can be confirmed.

A firing temperature lower than 600° C. yielded a peak, e.g. the peak ofLi₂CO₃ in use of lithium carbonate (Li₂CO₃) as the lithium source,showing a remaining raw material on the XRD chart of the burnt matter.In this case, a monophasic product is not yielded. On the other hand, afiring temperature as higher than 1000° C. did not yield the monophasicproduct, but yielded a high temperature phase other than an objectivecrystalline compound.

In lithium manganese oxide as described above and according to thepresent invention, the crystal structure is stabilized and the cycleproperty is improved in using it as the positive active material of thelithium secondary battery. Such improved cycle property appearsparticularly prominent in a large capacity battery using a largequantity of electrode active material. Thus, usage thereof isexemplified by a motor actuating power source for EV and HEV. However,the present invention can be naturally used for a small capacity batterysuch as a coin-type battery.

Various materials so far known can be used for other members (material)used in the lithium secondary battery, in which lithium manganese oxideaccording to the present invention is used as the positive activematerial. For example, amorphous carbon materials such as soft carbonand hard carbon and highly graphitized carbon materials such asartificial graphite and natural graphite can be used as a negativeactive material. Among them, it is preferable to use highly graphitizedcarbon materials of which lithium capacity is large.

The usable organic electrolytic solutions are those in which one or morefluorine compounds of lithium complex such as LiPF₆ and LiBF₄ or lithiumhalogenate such as LiClO₄ as electrolytes are dissolved in a singlesolvent or a mixed solvent of such organic solvent as carbonate esterssuch as ethylene carbonate (EC), diethyl carbonate (DEC), dimethylcarbonate (DMC), propylene carbonate (PC) and γ-butyrolactone,tetrahydrofurane, and acetonitrile.

The structures of the batteries are exemplified by various batteriessuch as a coin-type battery, in which a separator is arranged betweenthe positive active material and the negative active material is moldedin a plate shape to be filled with an electrolytic solution, andcylindrical and box-type batteries made by using an electrode body madeby winding through the separator or stacking around a positive plate,which is prepared by coating the positive active material over thesurface of a metal foil, and a negative plate, which is prepared bycoating the negative active material over the surface of a metal foil.

EXAMPLES

Subsequently, the examples of the present invention are described below.However, the present invention is not restricted to the followingexamples.

(Synthesis of Lithium Manganese Oxide)

As raw materials, powder of commercially available Li₂CO₃, MnO₂, TiO₂,MgO, and NiO were used, and weighed and mixed to make the compositionaccording to the Examples 1 to 3 and Comparative Example shown in Table1, followed by firing in an atmosphere under the conditions described inthe same Table 1. The samples of Examples 1 and 3 and the ComparativeExample were subjected to a pulverizing process to make an averageparticle size to 10 μm or smaller after the first firing. Subsequently,they were burnt under the second firing conditions according to thedescription of the Table 1 to yield finally the sample.

TABLE 1 First Second Sample burning burning name Composition conditioncondition Example 1 Li(Ni_(0.5)Ti_(0.5))_(0.1)Mn_(1.9)O₄ 650° C. × 10800° C. × 24 hours hours Example 2 Li(Ni_(0.5)Ti_(0.5))_(0.1)Mn_(1.9)O₄650° C. × 10 (None) hours Example 3 Li_(1.1)Mn_(1.9)O₄ 650° C. × 10 800°C. × 24 hours hours Comparative LiMn₂O₄ 650° C. × 10 800° C. × 24Example hours hours(Differential Thermal Analysis)

Differential thermal analysis of various lithium manganese oxide samplesobtained was conducted by using a thermobalance named TG-DTA Thermoflex(high temperature type; made by Rigaku Denki, K. K.) under theconditions shown in Table 2. Experimental procedures were the same asnormal ones applied by those skilled in the art.

TABLE 2 Temperature measurement range Room temperature-1200° C.Temperature controlling PtRh13%-Pt thermocouple Heater Platinum rhodiumMeasurement atmosphere In atmosphere Measurement sample wt 40 mg-50 mgSample container 4 mmφ × 3 mm Alumina made pan Reference sample Highpurity alumina powder for thermal analysis Temperature rising rate 5°C./min Sampling time Every 5 sec.(Preparation of a Battery)

Various lithium manganese oxide samples prepared were mixed withacetylene black powder, which is an adjuvant for electric conduction,and polyvinylidene fluoride, which is a binding agent, in a weight ratioof 50:2:3 to prepare the positive material. The positive material of0.02 g was subjected to press molding to make a disk shape with adiameter of 20 mm under a pressure of 300 kg/cm² to prepare the positiveelectrode. The coin cell was prepared by using this positive electrode,the electrolytic solution prepared by dissolving LiPF₆ as theelectrolyte in an organic solvent, in which ethylene carbonate anddiethyl carbonate were mixed in a equal volume proportion, to make aconcentration of 1 mol/L, the negative electrode made from carbon, andthe separator separating the positive from the negative electrode.

(Evaluation of Cycle Property)

The coin cell prepared was charged up to 4.1 V applying a constantcurrent and a constant voltage of 1C rate according to capacity of thepositive active material and discharged up to 2.5V by applying aconstant current of the same 1 Crate. These charge and discharge makeone cycle. The experiment was conducted for 100 cycles. The dischargecapacity in the 100^(th) cycle was divided by the first dischargecapacity to yield a proportion that was used for evaluation of the cycleproperty.

(Test Result)

FIGS. 1(a) and (b) show the results of differential thermal analyses ofthe Example 1 and the Comparative Example, respectively. For therespective samples, Table 3 shows the strength ratio (P₂/P₁ strengthratio) of the primary endothermal peak (P₁) to the secondary endothermalpeak (P₂) and the capacity proportion to the first discharge capacity at100 cycles. Here, for the P₂/P₁ strength ratio, the length of a normalfrom the apex of the endothermal peak to a line was sought by thefollowing steps: the point of start of the rise of respectiveendothermal peak was connected to the point of the end of the peak withthe line, this length of the line was regarded as the strength of theendothermal peak, and the P₂ strength was divided by the P₁ strengthobtained to yield the P₂/P₁ strength ratio.

TABLE 3 P₂/P₁ Discharge capacity strength proportion at 100 Sample nameComposition ratio cycles Example 1 Li(Ni_(0.5)Ti_(0.5))_(0.1)Mn_(1.9)O₄<0.1 0.90 Example 2 Li(Ni_(0.5)Ti_(0.5))_(0.1)Mn_(1.9)O₄ 0.5 0.88Example 3 Li_(1.1)Mn_(1.9)O₄ 0.9 0.81 Comparative LiMn₂O₄ 1.0 0.53Example

As shown in the Table 3, the P₂/P₁ strength ratio was 1 and thedischarge capacity proportion at 100 cycles was as small as 53% in theComparative Example, pointing out a problem in the cycle property. Onthe other hand, as shown in the results of the Examples 1 to 3, theP₂/P₁ strength ratio was under 1 and the discharge capacity proportionat 100 cycles increased according to a decrease in the P₂/P₁ strengthratio. Thus, the cycle property was certainly improved.

Such difference in appearance of such endothermal peaks, particularlythe minimal secondary endothermal peak may be due to a stabilizedcrystal structure to suppress release of oxygen and Li by heat. Improvedcycle property by stabilized crystal structure may be based on adecrease in irreversibly changeable part of crystal structure due tomoved Li ions according to charge and discharge.

In a comparison of the Examples 1 and 2 with Example 3, substitution ofa part of Mn by two other elements provides prominent improvement of thecycle property than substitution by one element. Particularly, it isshown that when P₂/P₁ strength ratio was 0.5 or less, a good cycleproperty was obtained. Further, the comparison of the Example 1 withExample 2 shows the good cycle property in many number of times offiring even in the same composition. This may be caused by evenness ofcomposition and improvement of crystallinity realized by increase in thenumber of times of firing.

As described before, the use of lithium manganese oxide spinel having astable crystal structure, which differs in thermal property fromconventional lithium manganese oxide spinel, gives an excellent cycleproperty to the lithium secondary battery of the present invention. Inother words, the lithium secondary battery of the present inventionprovides a prominent effect in realizing a high capacity and a longlife.

1. A lithium secondary battery, wherein lithium manganese oxide is usedas a positive active material, said lithium manganese oxide having acubic spinel structure of which strength ratio (P₂/P₁ strength ratio) ofa primary endothermal peak (P₁) appearing around 950° C. and a secondaryendothermal peak (P₂) appearing around 1100° C. in differential thermalanalysis, is 0.5 or less, said lithium manganese oxide having a formulaLi(M_(1(x1))M_(2(x2))M_(3(x3)) . . . M_(m(xm)))_(x)Mn_(2−x)O₄, whereinM₁ is Ti, M₂ is Li, and M₃ . . . M_(m) are metals selected from thegroup consisting of Fe, Ni, Mg, Zn, Co, Cr, Sn, P, V, Sb, Nb, Ta, Mo andW, wherein x is a substituted amount greater than zero, wherein X₁ isgreater than zero, wherein X₂ is greater than or equal to zero, whereinat least one of X₃, . . . and X_(m) is greater than zero, and wherein asum of X₁, X₂, X₃,. . . and X_(m) is
 1. 2. The lithium secondary batteryaccording to claim 1, wherein said lithium manganese oxide is yielded byfiring a mixture of salt(s) and/or oxide(s) of respective elementsadjusted to a given proportion in an oxidation atmosphere, under atemperature in the range of 650 to 1000° C. and for a duration between 5hours and 50 hours.
 3. The lithium secondary battery according to claim2, wherein said lithium manganese oxide is yielded by carrying out saidfiring at least twice or more.
 4. The lithium secondary batteryaccording to claim 3, wherein said lithium manganese oxide is yielded bygradually increasing a firing temperature as the number of times offiring increases.
 5. The lithium secondary battery according to claim 1,wherein X₂ is greater than 0.